2007 Outburst of 17P/Holmes: The albedo and the temperature of the dust grains

Based on optical and infrared observations, we study the albedo and the temperature of the dust grains associated with the spectacular 2007 outburst of Jupiter-family comet 17P/Holmes. We found that the albedo at the solar phase angle ∼ 16° was 0.03-0.12. While the color temperature around 3-4 μm was 360 ± 40K, the color temperature at 12.4 μm and 24.5 μm was ∼ 200K, which is consistent with that of a blackbody. We studied the equilibrium temperature of the dust grains at 2.44AU and found that the big discrepancy in the temperature was caused by the heterogeneity in particle size, that is, hotter components consist of submicron absorbing grains whereas colder components consist of large (≳1 μm) grains. The contemporaneous optical and mid-infrared observations suggest that the albedo and the temperature could decrease within ∼ 3 days after the outburst and stabilized at typical values of the other comets. We estimated the total mass injected into the coma by the outburst on the basis of the derived albedo and the optical magnitude for the entire dust cloud, and found that at least 4 × 10¹⁰kg (equivalent to a few meter surface layer) was removed by the initial outburst event. The derived mass suggests that the outburst is explainable by neither the exogenetic asteroidal impact nor water ice sublimation driven by solar irradiation, but by an endogenic energy source. We conclude that the outburst was triggered by the energy sources several meters or more below the nuclear surface.